EB/SM splitter heat recovery

ABSTRACT

Improved methods and related apparatus are disclosed for efficiently recovering the heat of condensation from overhead vapor produced during separation of various components of dehydrogenation reaction effluent, particularly in ethylbenzene-to-styrene operations, by the use of at least a compressor to facilitate azeotropic vaporization of an ethylbenzene and water mixture within a preferred range of pressure/temperature conditions so as to minimize undesired polymerization reactions.

FIELD OF THE INVENTION

The present invention relates to a low temperature heat recoverytechnique in the process of making styrene through dehydrogenation ofethylbenzene at elevated temperatures in the presence of steam.Specifically, this invention teaches methods of recovering the heat ofcondensation from the overhead vapor leaving the distillation columnwhich is used for separation of unreacted ethylbenzene from the styreneproduct (hereinafter referred to as the EB/SM splitter) together withrelated apparatus. Typically, this heat is rejected to atmospherethrough the use of cooling water or air fins and is therefore wasted.The EB/SM splitter typically has a heat removal requirement of between400 and 700 kcal/kg of styrene product, which represents a significantportion of the overall cost of styrene production. Recovery of asubstantial portion of this thermal energy dramatically improvesoperating economics and process efficiencies.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,171,449 teaches methods of recovering at least a portionof the heat contained in an EB/SM splitter overhead stream via use of acascade reboiler scheme in which the separation of ethylbenzene andstyrene is carried out in two parallel distillation columns operating atdifferent pressures, with the overhead of the high pressure columnproviding the heat required to reboil the low pressure column.

In contrast, U.S. Pat. No. 4,628,136 teaches a method of recovering theheat contained in the overhead of the EB/SM splitter by using thisstream to boil an azeotropic mixture of ethylbenzene and water, which,once vaporized, is subsequently transferred to the reaction system wheredehydrogenation of ethylbenzene to styrene takes place. The methoddescribed in the U.S. Pat. No. 4,628,136 patent, however, requires thatthe EB/SM splitter operate at a pressure that is sufficiently high as toenable the transfer of the azeotropic mixture of ethylbenzene and watervapor into the reactor system without the use of a compressor. Thispatent also specifies that the temperature difference between thecondensing EB/SM splitter overhead and the boiling azeotropic mixture ofethylbenzene and water should be in the range of between and 2 and 10°C. Given this temperature constraint, one can derive a relationshipbetween the pressure at which the azeotropic vaporization is takingplace and the required overhead pressure of the EB/SM splitter. Thisrelationship is presented graphically in FIG. 4.

As can be seen in the graph presented in FIG. 4, the method taught byU.S. Pat. No. 4,628,136 requires that the EB/SM splitter operate at anoverhead pressure of at least 200 mmHg in order for the azeotropicmixture to be transferred into the reactor system without the use of thecompressor. This is because the practical lower limit for the pressureat the inlet of the reactor system is of the order of 400 mmHg, and mayrange up to about 1100 mmHg, which must be increased by another 100 to200 mmHg in order to pass the azeotropic mixture of ethylbenzene andwater vapor through the heat exchange system (e.g., reactorfeed-effluent exchanger or a fired heater) which is needed to bring itto the required reaction temperature and to pass this stream into andthrough the reactor system. As a consequence of this limitation, themethod taught by U.S. Pat. No. 4,628,136 results in required operatingtemperatures for the EB/SM splitter which are significantly higher thanin a conventional process where no effort is made to recover heat fromthe overhead. Operation at such higher temperature and pressure,however, is more costly both in operational and capital costs.

The necessary increase in operating temperature and pressure which isrequired to practice the method of the U.S. Pat. No. 4,628,136 patentalso leads to an increase in the rate of styrene polymerization which isa direct yield loss. For uninhibited styrene monomer, the polymerizationrates approximately double for every 7 to 8° C. increase in temperature.In commercial practice, the method taught by U.S. Pat. No. 4,628,136results in operating the EB/SM splitter at temperatures on the order of20° C. to 30° C. higher than conventional technology. The net result iseither the need for increased dosage rates of costly polymerizationinhibitors or accepting an increased formation of undesired styrenepolymer (yield loss), or both, resulting in a substantial negativeimpact on the overall process economics. Furthermore, the close-couplingof the EB/SM splitter and the dehydrogenation reactor system operationsrequired to practice the method of the U.S. Pat. No. 4,628,136 patentmeans that an increase in pressure drop anywhere in the reaction system(as for example that which may be caused by fouling of heat exchangesurfaces or by catalyst attrition leading to higher pressure drop in thecatalyst beds) will require that the EB/SM splitter be operated undereven higher pressure and temperature conditions than usual, resulting instill further increases in polymerization inhibitor consumption, styrenepolymer byproduct, or both.

These and other deficiencies in or limitations of the prior art areovercome in whole or in part by the improved method and relatedapparatus of the present invention.

SUMMARY OF THE INVENTION

In a principal embodiment of the new invention described herein, it hasbeen found that the aforementioned limitations of the method taught byU.S. Pat. No. 4,628,136 can be overcome by use of a compressor. Using acompressor at one or more selected locations in the process flow schemerealizes a number of important and unexpected benefits over the priorart including: a) it allows the EB/SM splitter to operate at asubstantially lower pressure and temperature; b) it compensates for anyreasonable pressure drop increases in the reaction section; c) it allowsthe EB/SM splitter operating conditions to be set independently from thereaction section of the overall process; d) it allows higherdifferential temperatures between the condensing overhead and thevaporizing azeotrope, resulting in smaller heat transfer arearequirements; and e) it allows recovery of substantially all of theusable heat contained in the overhead stream.

The general concept of using of a compressor for transferring anazeotropic mixture of ethylbenzene and water vapor into thedehydrogenation reactor system was taught earlier by U.S. Pat. No.4,695,664. However, in the method taught in the U.S. Pat. No. 4,695,664patent, the azeotropic mixture of ethylbenzene and water is boiled byheat exchange with the reactor effluent rather than using the EB/SMsplitter overhead, as taught by this invention, to provide the necessaryheat. As a consequence of this difference, in the practice of the U.S.Pat. No. 4,695,664 patent the pressure of the azeotropic mixture shouldbe maintained at about 200 mmHg. Pressures higher than this areundesirable because of the need to operate the dehydrogenation reactorsat a higher pressure (requiring more catalyst and more steam to maintaincatalyst stability), while operating the system at pressures lower than200 mmHg makes compression costs prohibitively expensive. In contrast,the method of the present invention can be practiced at a higherazeotropic mixture pressure, in the range of about 150 to 600 mmHg,preferably about 250 to 390 mmHg, limited only by the polymerizationconsiderations in the EB/SM splitter.

Thus, the unique features of the methods and apparatus of the presentinvention allow the azeotropic vaporization of the EB/water mixture totake place in the pressure range of about 150 to 600 mmHg, preferably arange of about 250 to 390 mmHg, which largely falls outside theacceptable pressure ranges taught by prior art methods. In addition,other unexpected efficiencies and economies are realized with themethods and apparatus of this invention as described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a first embodiment of the presentinvention wherein a compressor unit is located in-line between thecondenser downstream of the fractionator and the dehydrogenationreactor.

FIG. 2 is a process flow diagram of an alternative embodiment of thepresent invention wherein a compressor unit is located in-line betweenthe fractionator and the condenser downstream of the fractionator.

FIG. 3 is a process flow diagram of a third embodiment of the presentinvention which utilizes two compressor units downstream of thefractionator.

FIG. 4 is a graphical representation of a relationship between thepressure at which the azeotropic vaporization takes place and therequired overhead pressure of the EB/SM splitter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The methods and apparatus of this invention pertain to catalyzedhydrocarbon dehydrogenation processes, for example the process ofmanufacturing styrene via dehydrogenation of ethylbenzene in thepresence of steam at elevated temperatures in a reactor system typicallycontaining an iron oxide based dehydrogenation catalyst.

A first embodiment of this invention, as applied to the manufacture ofstyrene by the above process, is illustrated in FIG. 1. In thisembodiment, a gaseous mixture 2 of ethylbenzene and steam is mixed withadditional steam 1 which has been preheated to a temperature oftypically between about 700 and 900° C. in a fired steam superheater101. The resulting mixture 3 is passed through a dehydrogenation system102 comprising one or more dehydrogenation reactors together with ameans of supplying heat to compensate for heat lost due to theendothermic nature of the dehydrogenation reaction. The reactors can beeither isothermal or adiabatic, and the heat can be added eitherdirectly (e.g., by passing the reaction mixture through a fired heateror through flameless distributed combustion tubes, as described forexample in U.S. Pat. Nos. 5,255,742 and 5,404,952, which patents areincorporated herein by reference), or indirectly, by contacting thereaction mixture with a heat carrying medium such as steam, molten saltor flue gas in a shell and tube heat exchanger. The dehydrogenationreaction is carried out at a temperature of between about 500 and 700°C., preferably between about 550 and 650° C., and at a pressure ofbetween about 0.3 and 2 atmospheres, preferably between about 0.3 and0.8 atmospheres, and preferably in the presence of a iron oxide baseddehydrogenation catalyst, examples of which include catalysts commonlyreferred to by their trade names of Styromax 3, Hypercat, and D-0239E,as is well-known in this art. The overall molar ratio of steam toethylbenzene in the reactor feed 3 is typically between about 5 and 15.Lower ratios are preferred because of reduced steam cost, reducedeffluent condensation cost, and investment savings resulting fromsmaller equipment. The minimum steam to ethylbenzene ratio at which theprocess can be carried out depends on a variety of factors, includingcatalyst stability and on metal structural temperature limits in thesteam superheater 101 and the dehydrogenation system 102.

The reactor effluent 4 is cooled in a feed/effluent heat exchanger 103where it exchanges heat with the relatively cold reactor feed 13. It isthen cooled further in a steam generator 104 and at least partiallycondensed in a condenser 105 using either air or cooling water as acooling medium (not shown). The partially condensed effluent flows intoa phase separator 106 where the dehydrogenation vent gas 5 is separatedfrom the liquids. The liquids coming from separator 106 are thendecanted into a hydrocarbon stream 7 and an aqueous condensate stream 6.The hydrocarbon stream 7, often referred to as a crude styrene stream,contains a mixture of styrene, unreacted ethylbenzene, and water/steam,as well as reaction byproducts such as benzene, toluene and various highboiling compounds which may include alpha-methylstyrene, divinylbenzene,and dicyclics (e.g., stilbene).

The crude styrene stream 7 is then typically processed in a series ofdistillation columns for separating out various light and heavyfractions. The first step in this process typically involves removingbenzene and toluene from the balance of the mixture, followed by asecond step in which unreacted ethylbenzene is recovered. Alternatively,ethylbenzene may be removed together with benzene and toluene in thefirst step, and then be separated from these lighter components in thesecond step. In either scheme, the last distillation step involvesseparation of styrene from the heavier components.

For the purposes of illustrating this invention in FIGS. 1, 2 and 3, wehave chosen to present the scheme in which the first step in processingcrude styrene stream 7 involves removal of ethylbenzene together withthe lighter components. It will be understood, however, that the methodsof the present invention are also applicable to the alternative schemediscussed above. In the scheme illustrated in FIG. 1, the crude styrenestream 7 is fed to a fractionator 107, which is preferably operatedunder vacuum. Operating the fractionator under vacuum is advantageous tothe process in general in that it lowers the temperature of bottomsstream 10 thereby decreasing the rate of styrene polymer formation, orreducing the amount of costly polymerization inhibitor 14 which must beadded to stream 7, or both. Typically, the fractionator 107 is designedto operate at an overhead pressure below 100 mmHg, which results in abottoms stream 10 at a temperature of less than 100° C.

In prior art processes in this field, the overhead vapor stream 8leaving the fractionator 107 is typically condensed in a condensersimilar to azeotropic vaporizer 108 but utilizing either cooling wateror air, which is then vented or disposed of without any heat recovery.When condensed in this manner as a step in a conventional process, thelatent heat of vaporization carried by the overhead vapor stream 8 istypically rejected to the atmosphere because the temperature of thisstream is too low for use in generating steam or to vaporizeethylbenzene. In accordance with the present invention, however, it hasnow been found that overhead vapor stream 8 can be condensed, and theheat of condensation can be used to vaporize an azeotropic mixture ofethylbenzene and water because such mixtures boil at temperaturessignificantly below the respective boiling points of the pure individualcomponents.

In accordance with the methods of this invention, therefore, a fractionof about 0.30-1.0, preferably about 0.50-0.80, of overhead vapor stream8 leaving the fractionator 107 is condensed by using it to boil amixture of ethylbenzene and water 17 in an azeotropiccondenser/vaporizer 108, which may be similar to the vaporizer describedin U.S. Pat. No. 4,628,136. Other types of vaporizers, such as thosedescribed in U.S. Pat. No. 4,695,664, can also be used in carrying outthe methods of this invention. U.S. Pat. Nos. 4,628,136 and 4,695,664are incorporated herein by reference. In prior art processes, such asthat taught by the U.S. Pat. No. 4,628,136 patent, the acceptabletemperature differential in the condenser between the condensingfractionator overhead vapor stream and the boiling azeotropic mixture isin the range of about 2-10° C., preferably about 6° C. By contrast, themethods and apparatus of the present invention can accommodate a largertemperature differential of about 10-30° C., preferably about 15-25° C.,between the condensing vapor and the boiling azeotropic mixture invaporizer 108, leading to additional process flexibility and realizingfurther efficiencies.

A portion 9 of the condensed overhead, preferably a predominant portionof the condensed overhead, leaving the azeotropic vaporizer 108 isreturned to the fractionator 107 as reflux stream 16, and the remainder15 is directed to another downstream fractionator (not shown) whereunreacted ethylbenzene is recovered from lighter components. Thisrecovered ethylbenzene stream is then mixed with fresh ethylbenzene toform a combined ethylbenzene feed 11 which is returned to the system. Asshown in FIGS. 1, 2 and 3, in preferred embodiments of this invention aportion of the aqueous reactor condensate 6 can be split off from themain stream and added to the combined ethylbenzene feed 11, and theresulting azeotropic ethylbenzene/water mixture 17 is then directed tothe azeotropic vaporizer 108 to be boiled with heat drawn from thefractionator overhead vapor stream 8. In a further preferred embodimentof this invention, the molar ratio of water to ethylbenzene in theethylbenzene/water mixture is between about 4-12, preferably about 6-10.

The size of the vaporizer 108 will be inversely proportional to thetemperature difference between the condensing overhead vapor 9 comingfrom vaporizer 108 and the boiled azeotropic mixture of ethylbenzene andwater 12 also coming from vaporizer 108, as determined by theirrespective pressures. In a prior art system, such as that described inU.S. Pat. No. 4,628,136, the pressure of the azeotropic mixture ofethylbenzene and water must be substantially above the pressure existingat the inlet to the dehydrogenation reactor section 102, typically inthe range of about 400-1100 mmHg, to allow this stream to pass throughthe feed effluent exchanger 103 where it is preheated prior to beingmixed with superheated steam 1 from stream superheater 101. As aconsequence, the fractionator 107 must be operated at a pressure suchthat the condensing overhead temperature is at least 2° C., andpreferably at least 6° C. or more, higher than the temperature of theazeotropic mixture of ethylbenzene and water going to heat exchanger103. As a result, the temperature of bottoms stream 10 coming fromfractionator 107 will necessarily be significantly higher than theoptimal temperature. This higher temperature of bottoms stream 10 leadsto increased formation of undesirable styrene polymer and/or requires ahigher dosing rate of the costly polymerization inhibitor 14, or both.

In the practice of the present invention as illustrated in FIG. 1,however, this problem is overcome by employing an in-line compressorunit 109 between vaporizer 108 and heat exchanger 103 in order tocompress the azeotropic mixture of ethylbenzene and water 12 to thepressure required for it to pass to and through the dehydrogenationreaction system 102. As a result of this innovation, the operatingtemperature used for fractionator 107 is decoupled from downstreampressure considerations. Fractionator 107 can thus be operated at lower,more optimal temperatures and pressures, for example at a pressure belowabout 200 mmHg, preferably in the range of about 70-170 mmHg, leading tolower temperatures of fractionator bottoms stream 10, which in turnminimizes undesirable polymerization of styrene in bottoms stream 10 andreduces the consumption of expensive polymerization inhibitor 14. Evenwith the methods and apparatus of the present invention, however, atleast a small addition of a polymerization inhibitor 14 to stream 7 willgenerally be desirable to still further reduce the loss of styreneproduct. Such state-of-the-art polymerization inhibitors include thosetaught in U.S. Pat. Nos. 6,300,533; 6,287,483; 6,222,080; and 5,659,095,which patents are incorporated herein by reference.

In another embodiment as illustrated in FIG. 2, the fractionatoroverhead vapor stream 8 is compressed using compressor 110, but theazeotropic mixture of ethylbenzene and water 12 is not separatelycompressed. This embodiment of the present invention also facilitatesdecoupling the operating temperature of fractionator 107 from thepressure of the azeotropic ethylbenzene/water mixture. Becausefractionator overhead vapor stream 8 coming out of compressor 110 is ata higher pressure, vaporizer 108 can correspondingly be operated at ahigher pressure resulting in a higher pressure boiled azeotropicethylbenzene/water mixture coming out of vaporizer 108.

In a yet another embodiment of this invention as illustrated in FIG. 3,both the fractionator overhead vapor stream 8 and the azeotropic mixtureof ethylbenzene and water 12 are compressed, respectively, withcompressor units 110 and 109. This embodiment of the present inventionalso facilitates decoupling the operating temperature of fractionator107 from the pressure of the azeotropic ethylbenzene/water mixture. Incomparison with the embodiments of FIGS. 1 and 2, however, theembodiment of FIG. 3 also decouples the temperature/pressure conditionsin vaporizer 108 from the temperature/pressure conditions infractionator 107, thereby creating still additional operatingflexibility. In this embodiment, vaporizer 108 may be operated at anypressure (and corresponding temperature) between thetemperature/pressure of fractionator 107 and the temperature/pressurerequired to properly feed the azeotropic ethylbenzene/water mixture todehydrogenation reaction system 102.

All of the embodiments illustrated in FIGS. 1, 2 and 3, however, sharethe same advantages over the method described in U.S. Pat. No.4,628,136, wherein no compression is used, in that they allow thefractionator 107 to be operated at a relatively low temperature andpressure, substantially the same as that of conventional processes,thereby minimizing styrene polymer byproduct, while also minimizingusage of expensive polymerization inhibitors, and while still recoveringsubstantially all of the useful heat from the fractionator overheadvapor stream. Thus, the U.S. Pat. No. 4,628,136 patent teaches apreferred pressure of 15 psia for the azeotropic mixture of ethylbenzeneand water, and a minimum (and preferred) pressure of 280 mmHg for thefractionator overhead stream, leading to a fractionator bottomstemperature of 125° C., which results in a high polymer make despite theuse of a polymerization inhibitor.

By comparison, the methods and apparatus of the present inventionutilize a preferred pressure of about 250-390 mmHg (5-7.8 psia) for theazeotropic mixture of ethylbenzene and water, and a preferred pressureof about 50-170 mmHg for the fractionator overhead stream (beforecompression), leading to a fractionator bottoms temperature of about105° C. at the preferred overhead stream pressure, which reduces thepolymer make by a factor of 4 relative to the polymer make in theprocess taught by the U.S. Pat. No. 6,628,136 patent. This illustrativecomparison at preferred operating parameters clearly demonstrates theunexpected superiority of the present invention over the method taughtby the U.S. Pat. No. 4,628,136 patent.

It will be apparent to those skilled in the art that other changes andmodifications may be made in the above-described apparatus and methodsfor low temperature heat recovery from the overhead vapor from the EB/SMsplitter in styrene manufacture without departing from the scope of theinvention herein, and it is intended that all matter contained in theabove description shall be interpreted in an illustrative and not alimiting sense.

1. In a method of manufacturing styrene by dehydrogenation ofethylbenzene in the presence of steam at elevated temperatures in areactor system containing a dehydrogenation catalyst, the improvementcomprising the steps of: (a) separating the unreacted ethylbenzene fromthe crude styrene by fractionation in an ethylbenzene-styrenefractionator carried out at an overhead pressure below about 200 mmHg inthe presence of a polymerization inhibitor; (b) condensing theethylbenzene overhead vapor from the fractionator in an azeotropicvaporizer to provide heat for boiling a reactor feed consistingessentially of an azeotropic mixture of ethylbenzene and water; and, (c)compressing the vaporized reactor feed, the overhead vapor from thefractionator, or both to obtain an azeotropic mixture of ethylbenzeneand water at a suitable pressure for feeding to the reactor system.
 2. Amethod according to claim 1, further wherein the ethylbenzene-styrenefractionator is operated at an overhead pressure of between about 50-170mmHg.
 3. A method according to claim 1, further wherein the azeotropicmixture is boiled at a pressure of between about 250-390 mmHg.
 4. Amethod according to claim 1, further wherein the temperature differencebetween the condensing overhead vapor and the boiling azeotropic mixtureof ethylbenzene and water in the azeotropic vaporizer is between about15-25° C.
 5. A method according to claim 1, further wherein the fractionof overhead ethylbenzene vapor condensed in the azeotropic vaporizer isbetween 0.30 and 1.0.
 6. A method according to claim 1, further whereinthe water in the azeotropic mixture is derived from process condensate.7. A method according to claim 1, further wherein the molar ratio ofwater to ethylbenzene in the reactor feed is between about 4 and
 12. 8.A method according to claim 1, further wherein the pressure at the inletto the reactor system is between about 400-1100 mmHg.
 9. A methodaccording to claim 1, further wherein said dehydrogenation catalystconsists essentially of an iron oxide based dehydrogenation catalyst.10. A method according to claim 1, further wherein said fractionation iscarried out under vacuum.
 11. A method according to claim 1 wherein step(c) comprises compressing only the vaporized reactor feed.
 12. A methodaccording to claim 1 wherein step (c) comprises compressing only theoverhead vapor from the fractionator.
 13. A method according to claim 1wherein step (c) comprises compressing both the vaporized reactor feedand the overhead vapor from the fractionator.
 14. In a method ofdehydrogenation of an alkylaromatic compound in the presence of steam atelevated temperatures in a reactor system containing a dehydrogenationcatalyst, the improvement comprising the steps of: (a) separatingunreacted alkylaromatic compound from the crude product by fractionationin a fractionator carried out at an overhead pressure below 200 mmHg inthe presence of a polymerization inhibitor; (b) condensing the overheadvapor from the fractionator to provide heat for boiling a reactor feedconsisting essentially of an azeotropic mixture of the alkylaromaticcompound and water; and, (c) compressing the vaporized reactor feed, theoverhead vapor from the fractionator, or both to obtain an azeotropicmixture of the alkylaromatic compound and water at a suitable pressurefor feeding to the reactor system.